Why not use the Wonders of the World? What better symbol than the ultimate achievement of man's engineering prowess?

Great Pyramid of Giza

Hanging Gardens of Babylon

Temple of Artemis

Statue of Zeus

Mausoleum at Helicarnassus

Colossus of Rhodes

Lighthouse of Alexandria

Pros: There are seven. This means that one can be allocated for each of the seven classical planets and mechanical arts.

Cons: There are seven, so this removes the ability to use the Wonders as a theme for a game. There also aren't any obvious tokens which can be used. You'd have to use some sort of custom token. The wonders also have a lot of baggage and/or vague meanings. Finally, unlike the playing cards in particular, they are hard to combine for additional complexity (suit + numbers).